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<title>s6: the ftrigw library interface</title>
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<h1> The <tt>ftrigw</tt> library interface </h1>
<p>
The <tt>ftrigw</tt> library provides an API for notifiers, i.e.
programs that want to regularly announce what they're doing.
</p>
<p>
Notifiers should create a fifodir in a hardcoded place in the
filesystem, and document its location. Listeners will then be
able to subscribe to that fifodir, and receive the events.
</p>
<h2> Programming </h2>
<p>
Check the <tt>s6/ftrigw.h</tt> header for the
exact function prototypes.
</p>
<h3> Creating a fifodir </h3>
<pre>
char const *path = "/var/lib/myservice/fifodir" ;
gid_t gid = -1 ;
int forceperms = 0 ;
int r = ftrigw_fifodir_make(path, gid, forceperms) ;
</pre>
<p>
<tt>ftrigw_fifodir_make</tt> creates a fifodir at the <tt>path</tt> location.
It returns 0, and sets errno, if an error occurs.
It returns 1 if it succeeds. <br />
If a fifodir, owned by the user, already exists at <tt>path</tt>, and
<tt>forceperms</tt> is zero, then <tt>ftrigw_fifodir_make</tt> immediately
returns a success. If <tt>forceperms</tt> is nonzero, then
it tries to adjust <tt>path</tt>'s permissions before returning.
</p>
<p>
If <tt>gid</tt> is negative, then <tt>path</tt> is created "public".
Any listener will be able to subscribe to <tt>path</tt>.
If <tt>gid</tt> is nonnegative, then <tt>path</tt> is created "private".
Only processes belonging to group <tt>gid</tt> will be able to
subscribe to <tt>path</tt>.
</p>
<h3> Sending an event </h3>
<pre>
char event = 'a' ;
r = ftrigw_notify(path, event) ;
</pre>
<p>
<tt>ftrigw_notify</tt> sends <tt>event</tt> to all the processes that are
currently subscribed to <tt>path</tt>.
It returns -1 if an error occurs, or the number of successfully notified
processes.
</p>
<h3> Cleaning up </h3>
<p>
When stray KILL signals hit <a href="s6-ftrigrd.html">s6-ftrigrd</a> processes,
1. it's a sign of incorrect system administration, 2. they can leave
unused named pipes in the fifodir. It's the fifodir's owner's job, i.e.
the notifier's job, to periodically do some housecleaning and remove
those unused pipes.
</p>
<pre>
r = ftrigw_clean(path) ;
</pre>
<p>
<tt>ftrigw_clean</tt> cleans <tt>path</tt>. It returns 0, and sets errno,
if it encounters an error. It returns 1 if it succeeds.
</p>
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